Hard to explain water temperatures to people from Texas or Africa, but I know you know exactly how it is :)
It's been years since I swam in the North Sea, I was a teenager last time, rather you than me! Jellyfish are horrible nasty things, I have been stung before.
I think I have inherited that boy scout gene from my grandmother. She loved swimming in cold water, crime novels and football (watching it not playing it).
The jellyfish that are there now are the harmless ones, but the year before I was stung pretty badly in the face by one of the red ones. We call them firemen and they are most common late in the season when the water gets colder... but that boy scout gene makes me swim even then.